Twilight Sparkle and Shining Armor share more than mere sibling affection, yet the Princess of Love seems quite happy with the arrangement.

Simple Clopfic

Warning: contains incest (1,2) and female domination (3)

Chapter 1: Shining receives an unexpected, though not unwelcome, guest.
Chapter 2: Cadence demonstrates the benefits of sharing.
Chapter 3: Pink princess takes black pawn... but black pawn isn't all she appears.